The invention relates to elastomeric compositions suitable for use in the tread portion of a pneumatic tire, to processes of producing them, to unvulcanized elastomeric compositions containing them and to tires containing the elastomeric compositions in a vulcanized state.
Elastomeric compositions suitable for use in the tread portion of a pneumatic tire should yield tires having a low rolling resistance for low fuel consumption, a high wet grip Performance for good running safety of automobiles and a high resistance against abrasion. Two general classes of tread compounds can be distinguished:
(a) those giving high wet grip performance combined with high heat build-up characteristic, relatively high rolling resistance, a low resistance against abrasion and having a glass transition temperature above minus 55.degree. C., and
(b) those giving relatively low wet grip performance combined with low heat build-up characteristics, relatively low rolling resistance, a high resistance against abrasion and having a glass transition temperature below minus 65.degree. C.
From the above it follows that it is difficult to combine a low rolling resistance with a high wet grip performance.
In U.S. Pat. No. 4,396,743 elastomeric compositions are described which mainly consist of a rubber mixture consisting of 20-95 parts by weight of a styrene-butadIene block copolymer and 80-5 parts by weight of a diene rubber; the styrene-butadiene block copolymer consists of a random copolymer block (A) and a random copolymer block (B), block (A) having a bonded styrene content of 20-50% by weight and a vinyl content in the butadiene unit of 40-75% by weight and block (B) having a bonded styrene content of not higher than 10% by weight and a vinyl content in the butadiene unit of 20-50%. The two blocks in this known composition are compatible with each other. As a result, the two blocks are complete-y compatibilized during vulcanization, the tan .delta.-temperature curve of the block copolymer having a broad width.
When the bonded styrene content in block (A) is higher than 50% by weight a rubber composition containing such a block copolymer is poor in rolling resistance.
In British patent application No. 2,158,076 elastomeric compositions are described which contain a styrene-butadiene block copolymer containing a random copolymer block (A) and a polybutadiene block (B), block (A) having a content of bound styrene of 10-80% by weight and a vinyl content in the butadiene unit of 30-70%, the content of vinyl bonds in block (B) being not more than 60%, each of the blocks (A) and (B) being included in the block copolymer in an amount of at least 20% by weight, the average vinyl content in block (A) being higher by at least 5% than that in block (B), block (A) having a glass transition temperature higher by at least 30.degree. C. than that of block (B) and satisfying three further requirements. According to this specification a vinyl content in the copolymer block (A) of less than 30% unfavorably affects the difference in glass transition temperatures between the copolymer blocks (A) and (B) and unsatisfactorily affects the balance between the rolling resistance and the wet grip performance. The rolling resistance is expressed as the loss tangent tan .delta. determined at a temperature of 50.degree. C. and the wet grip performance as tan .delta. determined at a temperature of 0.degree. C. For an explanation of the loss tangent reference is made to "Handbook of Plastics Test Methods", edited by R. P. Brown, 1981, Chapter Nine.